1. Field of the Invention
The present invention relates to a backlight unit and a method of driving the same, which uses a white light emitting diode (LED) as a light source and can achieve high efficiency and reliability in controlling a color temperature.
2. Description of the Related Art
Recently, with the rapid development of the semiconductor industry, products have been reduced in size and weight, and the performance thereof has become much stronger. Cathode ray tubes (CRT) which are widely used in information display devices have a number of advantages in terms of performance and price. However, the CRTs have a disadvantage in terms of size and portability. To overcome such a disadvantage, liquid crystal displays (LCD) are proposed as one measure. The LCDs have a number of advantages in terms of size, weight, power consumption. Therefore, the LCDs are being considered to be a substitute which can overcome the disadvantage of the CRTs. Currently, the LCDs are mounted on almost all information processing equipments which require display devices.
LCDs are referred to as display devices which apply a voltage to a specific molecular arrangement of liquid crystal to change into another molecular arrangement and converts changes in optical properties of liquid crystal cells, which emit light through such a molecular arrangement, into visual changes. That is, the LCDs use modulation of light caused by the liquid crystal cells. The optical properties of the liquid crystal cells include a birefringence property, optical rotary power, dichroism, a light scattering property and so on.
Such an LCD is a passive element which does not emit light. Therefore, the LCD illuminates an LCD panel by using a backlight unit attached on a rear surface of the LCD panel. The light transmittance of the LCD panel is adjusted by an applied electrical signal. Accordingly, a still image or moving image is displayed on the LCD panel.
In the backlight unit which supplies light to the LCD panel, a cold cathode fluorescent lamp (CCFL) has been widely used. Recently, however, various types of display devices such as a portable device, a field sequential color LCD and so on and backlight units using an LED are being widely used through an enhanced driving method.
The LED is a point source of light having a smaller light emission area than the CCFL. Light emitted from the LED is incident on a side surface of a light guide plate which guides a light path, and the light guide plate changes the light emitted from the LED into a surface source of light to supply to the LCD panel.
In general, to represent a natural color, the LED uniformly controls the luminance of three kinds of red, green, and blue LEDs such that white chromaticity coordinates are adjusted. At this time, the luminance of each LED should be controlled in such a manner that the luminance of light emitted from the backlight unit and the white chromaticity coordinates are adjusted. For this, a voltage control method is used, in which a constant voltage is applied to the LED and the time span of the applied voltage is adjusted so as to control the luminance and the white chromaticity coordinates.
However, when the luminance and the white chromaticity coordinates are controlled by the above-described method, a constant voltage is applied to each LED, and a pulse width of the constant voltage is modulated so as to supply power to the LED. Then, a time span where the LED is lit is adjusted. Therefore, as the pulse width increases, the LED itself is deteriorated. Accordingly, the life span of the LED is reduced, and the luminance is reduced. Then, the white chromaticity coordinates become unstable.
To solve such a problem, Korea Patent Laid-Open Publication No. 2007-44885 discloses a technique in which when the luminance and color temperature of each LED is changed depending on a temperature, a sensor detects the color temperature of each LED, and a controller attached to each LED changes an applied current of the LED such that the luminance and white chromaticity coordinates of the LED is uniformly controlled.
However, when the backlight unit is formed using only a white LED composed of a phosphor and a blue or near ultraviolet LED, not three kinds of red, green, and blue LEDs, the temperature of a light emitting section continuously changes until it reaches a steady state from when power is applied to the backlight unit, that is, when the backlight unit is driven. Therefore, although the sensor capable of detecting a color temperature is provided, it is impossible to continuously control the color temperature. Accordingly, it is difficult to uniformly maintain the white chromaticity coordinates, even though the luminance can be constantly maintained.